Purpose:
To assess the diagnostic performance of diffusion-weighted imaging (DWI) for the discrimination of complete responder (CR) from the non-complete responder (n-CR) in patients with locally advanced rectal cancer (LARC) undergoing chemotherapy and radiation (CRT).

Material and methods:
Between December 2009 and January 2014, 32 patients (33 lesions: one patient had two synchronous lesions) were enrolled in this retrospective study. All patients underwent a pre- and post-CRT conventional MRI study completed with DWI. For both data sets (T2-weighted and DWI), the pre- and post-CRT tumour volume (VT2; VDWI) and the tumour volume reduction ratio (∆V%) were determined as well as pre- and post-CRT apparent diffusion coefficient (ADC) and ADC change (∆ADC%). Histopathological findings were the standard of reference. Receiver operating characteristic (ROC) curves were generated to compare performance of T2-weighted and DWI volumetry, as well as ADC.

Results:
The area under the ROC curve (AUC) revealed a good accuracy of pre- and post-CRT values of VT2 (0.86; 0.91) and VDWI (0.82; 1.00) as well as those of ΔVT2% (0.84) and ΔVDWI% (1.00) for the CR assessment, with no statistical difference. The AUC of pre- and post-CRT ADC (0.53; 0.54) and that of ΔADC% (0.58) were significantly lower.

Conclusions:
Both post-CRT VDWI and ΔVDWI% (AUC = 1) are very accurate for the assessment of the CR, in spite of no significant differences in comparison to the conventional post-CRT VT2 (AUC = 0.91) and ΔVT2% (AUC = 0.84). On the contrary, both ADC and ΔADC% values are not reliable.